Method, a computer program product, and a carrier for indicating one-way latency in a data network

ABSTRACT

Disclosed herein is a method, a computer program product, and a carrier for indicating one-way latency in a data network (N) between a first node (A) and a second node (B), wherein the data network (N) lacks continuous clock synchronization, comprising: a pre-synchronisation step, a measuring step, a post-synchronisation step, an interpolation step, and generating a latency profile. The present invention also relates to a computer program product incorporating the method, a carrier comprising the computer program product, and a method for indicating server functionality based on the first aspect.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 14/194,885, filed Mar. 3, 2014, which is acontinuation of U.S. patent application Ser. No. 13/494,652, filed Jun.12, 2012, now U.S. Pat. No. 8,705,577, which is a continuation of U.S.patent application Ser. No. 11/662,646, filed Mar. 13, 2007, now U.S.Pat. No. 8,218,576, which is a U.S. national stage of InternationalApplication No. PCT/SE2005/001307, filed Sep. 9, 2005, which claims thebenefit of Sweden Patent Application No. 0402280-2, filed Sep. 22, 2004,all of which are incorporated herein in their entirety.

TECHNICAL FIELD

According to a first aspect, the present invention relates to a methodfor indicating one-way latency in a data network without continuousclock synchronization between a first node and a second node.

According to a second aspect, the present invention relates to acomputer program product incorporating the first aspect.

According to a third aspect, the present invention relates to a carriercomprising the computer program product.

According to a fourth aspect, the present invention relates to a methodfor indicating server functionality based on the first aspect.

BACKGROUND OF INVENTION

In the art, it is possible to achieve one-way real-time latencymeasurement by using synchronized clocks, such as GPS technology. It ispossible to use the standard NTP protocol to achieve a level ofsynchronization between nodes, as described in Mills, D “Network TimeProtocol (Version 3) specification, implementation and analysis”, IETFRFC 1305, University of Delaware, March 1992. However, the NTP mechanismdoes not have a very high accuracy.

In US2003/0048811 A1, with the title “Methods, systems and computerprogram products for synchronizing clocks of nodes on a computernetwork”, discloses an algorithm for clock synchronization between twonodes using virtual clocks, a generalization of the clocksynchronization for many nodes, and using many round-trip-delays tocompute an average one-trip delay. A key feature of the inventiondescribed in the US document, is that, each node manages a virtual clockfor every other node it synchronizes with.

SUMMARY OF INVENTION

According to the present invention and the first aspect, a method forindicating one-way latency in a data network, without continuous clocksynchronization, between a first node and a second node is disclosed.The method comprises:

-   -   a pre-synchronisation step,    -   a measuring step,    -   a post-synchronisation step,    -   an interpolation step, and    -   generating a latency profile.

Based on the present invention, also a computer program product loadableinto the internal memory of a computer, comprising software codeportions for performing the method, a carrier comprising the computerprogram product, and a method for indicating server functionality aredisclosed. This implies that the present invention is applicable whenmeasuring or monitoring, qualities of a server.

The present invention according to the present invention presents anumber of advantages in relation to prior art. For instance, theaccuracy of the measurements of the present invention is higher than theaccuracy of the NTP mechanism. The present invention deals with a methodfor making one-way real-time latency measurements with a high precisionbetween nodes connected by a message-passed network where clocksynchronization of that precision is not available, which is in contrastto the prior art briefly discussed above. The method may also presentindividual per-packet latency values. The present invention performshigh precision latency measurements of packets travelling between twonodes over a limited period of time. Since latencies may be asymmetric,round-trip estimation may not be used, but it must be relied on absoluteand synchronous time. Also, continuous clock synchronization does nothave to established, nor maintained. Instead, the present inventionmakes two synchronizations (before and after) with the single goal tointerpolate the measurement results.

In a preferred embodiment, the pre-synchronisation step comprisessending a predetermined message from the first node to the second node.Then, at the first node, a predetermined message sent by the second nodeis received. The next step is to calculate an offset according to((T₂−T₁)+(T₃−T₄))/2, where T₁ is the sending time from the first node,T₂ is the receiving time at the second node, T₃ is the sending time atthe second node, and T₄ is the receiving time at the first node.Thereafter, a clock difference is set to the offset. Also, an absoluteclock is set to T₄.

In a preferred embodiment, the pre-synchronisation step comprisessending the predetermined message N times, and the predetermined messageis received N times. However, there may be cases where all N messagesare not received. In such cases, there will be gaps in thecorrespondence of the data sent and received. This may be handled by notusing the measured values associated to the missing messages. Then N, ora number less than N, round-trip-time items are generated. This is doneaccording to T_(i,4)−T_(i,1)−(T_(i,3)−T_(i,2)), where i is in theinterval [1. . . N]. N offset items, or a number of offset items lessthan N, as described above is generated. The minimum round-trip-timeitem is retrieved and the clock difference is set to the offset relatedto the minimum round-trip-time item, and the absolute clock is set toT_(i,4) for the i having the minimum round-trip-time item.

In a preferred embodiment, the method further comprises the step ofmeasuring the overhead for making measurements at the sender.

In a preferred embodiment, the method further comprises the step ofmeasuring the overhead for making measurements at the receiver.

In a preferred embodiment, the measuring step comprises sending apredetermined message from the first node to the second node and storingthe sending time for the sending. The predetermined message is receivedat the second node, the receiving time for the receiving is stored.

In a preferred embodiment, the measuring step comprises sending apredetermined message from the first node to the second node N timeschronologically equidistantly, and the sending time for each sending isstored. The predetermined message is received at the second node, andthe receiving time for each receiving is stored. Hopefully, the messageis received N times at the second node. Otherwise this may be handled asdescribed above.

In a preferred embodiment, wherein the post-synchronisation step isconstituted by the pre-synchronisation step as defined above.

In a preferred embodiment, the interpolation step comprises calculatingthe one-way latency in the data network between a first node and asecond node according to the following relations:rate bias=(offset of the post-synchronisation step−offset of thepre-synchronisation step)/(absolute clock for post-synchronisationstep−absolute clock for the pre-synchronisation), andlatency=the time at which the second node received the predeterminedmessage−(the time at which the first node sent the predeterminedmessage+the offset of the pre-synchronisation step+(the time at whichthe first node sent the predetermined message−the absolute clock of thepre-synchronisation step)).

In a preferred embodiment, the interpolation step comprises calculatingthe one-way latency in the data network between a first node and asecond node according to the above for the messages sent between thefirst and second nodes.

In a preferred embodiment, the method further comprises an overhead termcorresponding to the overhead for making measurements at at least one ofthe first and second nodes, and the one-way latency in the data networkbetween a first node and a second node is calculated according to:latency=the time at which the second node received the predeterminedmessage−(the time at which the first node sent the predeterminedmessage+the offset of the pre-synchronisation step+ratebias(the time atwhich the first node sent the predetermined message−the absolute clockof the pre-synchronisation step))−the overhead term.

In a preferred embodiment, in the interpolation step, the one-waylatency in the data network between a first node and a second node iscalculated according to the above for the N messages sent between thefirst and second nodes. Alternatively in case all N messages were notreceived, this is done for the received messages.

It also lies within the scope of the present invention that it ispossible to operate in relation to more nodes than a single one. Ofcourse, the present invention may be used to operate against a pluralityof nodes.

BRIEF DESCRIPTION OF DRAWINGS

In FIG. 1, an embodiment of two nodes, A and B, interconnected by anetwork N are schematically depicted.

In FIG. 2, an embodiment of an architecture of a node is schematicallydepicted.

In FIG. 3, an embodiment of a network module is schematically depicted.

In FIG. 4, an embodiment of a requestor node pre-synchronizationflowchart is schematically depicted.

In FIG. 5, an embodiment of a responder node pre-synchronizationflowchart is schematically depicted.

In FIG. 6, an embodiment of a flowchart of requesting node in themeasurement phase is schematically depicted.

In FIG. 7, an embodiment of a flowchart of the responding node in themeasurement phase is schematically depicted.

In FIG. 8, an embodiment of a flowchart of the interpolation method isschematically depicted.

DESCRIPTION OF PREFERRED EMBODIMENTS

In a first embodiment, a system with two nodes A and B interconnected bya communication network N is depicted in FIG. 1. The nodes communicateby sending messages (packets) over the network N. A measurement isperformed from a node A to a node B, where A is called a requestingnode, and B is called a responding node. Each node may work both as arequesting node and a responding node. A node can also performmeasurements with more than one other node. For example, A can perform ameasurement with a third node C (not disclosed in FIG. 1) at the sametime. The network N may be an inter-network running the IP protocol.This enables any nodes with an IP-interface and an IP protocol stack tocommunicate with each other over N.

In FIG. 2, an embodiment of a node is shown. The computer node isequipped with a network interface card that can communicate using IP.Such a node has a CPU, memory buses, disks, etc, that enables it tooperate as a computer. The node runs an operating system, in which thesystem software can be implemented. This embodiment is implemented as asoftware module running in an operating system of such a node.

In FIG. 3, an embodiment of a network module is shown. The softwaremodule implementing the method described in this document needs to haveaccess to a network module. The network module shown in FIG. 3 typicallyconsists of a network interface card, a device driver, an IP stack and asocket API. The network interface card enables the node to physicallyconnect to an access network. The device driver contains softwareenabling the IP stack to access the network services on the networkinterface card. The IP stack contains full implementation of thecommunication protocols that enables the node to communicate over theinternet. This may be the set of protocols referred to as TCP/IP. Thesocket API is a functional interface that the system module can accessin order to send and receive packets to and from the network.

In an embodiment, a system module implementing the invention may beimplemented as a user application in an operating system. It requires asocket API to access the network in order to send and receive packetsover the network.

The nodes communicate with messages over the network. There are twokinds of messages:

-   -   Synchronization messages    -   Measurement messages

Both types of messages may be encapsulated over the IP protocol usingthe UDP/IP transport protocol or some other non-reliable datagramservice. In an embodiment, both types of messages are encoded with theRTP protocol.

A synchronization message is either a request (syncreq) or response(syncresp). The request message is sent by the requesting node andreceived by a responding node. A response is sent by a responding nodewhen it receives a syncreq message. The syncresp message is received bythe requesting node.

The syncreq message contains the following fields: a sequence number anda time-stamp T1.

The syncresp message contains the following fields: a sequence numberand three timestamps: T1, T2, and T3.

The semantics of the message fields are as follows:

-   -   Sequence number—The requesting node sets the sequence number        incrementally (0, 1, 2, etc). The responder copies the sequence        number from a syncreq to a syncresp message. The sequence number        is used to detect packet loss, reordering or duplications on the        network.    -   Timestamp T1. The time when the syncreq message was sent by the        requesting node.    -   Timestamp T2. The time when the syncreq message was received by        the responding node.    -   Timestamp T3. The time the syncresp message was sent by the        responding node.

The measurement messages are sent from the requesting node to theresponding node only. The measurement message contains a sequence fieldand a timestamp field T1.

The semantic of the message fields are as follows:

-   -   The sequence number. The requesting node sets the sequence        number incrementally (0, 1, 2, etc).    -   Timestamp T1. The time when the measurement message was sent by        the requesting node.

Now referring to the inventive method, both nodes have high accuracyclocks that are not synchronized with each other. High accuracy meansthat they are linear with respect to each other over a limited timeperiod on the order of minutes, and that they have high resolution, atleast to the level of 1 microsecond. That is, the clocks have differentrates, but the rate difference is constant over time.

The method is divided into five steps:

-   -   P1—Synchronization1    -   P2—Measurement    -   P3—Synchronization2    -   P4—Interpolation and    -   Generating a latency profile.

In Table 1 below an embodiment of constants used to parameterise themethod are given. The values given to the constants are merely anexample; the method can be used also for other values.

TABLE 1 Constant name Description Example values SNR Number of syncreqmessages sent. 50 NM Number of measurement messages 10000 sent. DT Delaybetween sending of 20 ms measurement messages.

In Table 2 below, variables used in this method are explained.

TABLE 2 Variable name Description NSREQ Number of syncreq messages sent.NSRESP Number of syncresp messages received. T1 Time when message wassent by requesting node. T2 Time when message was received by respondingnode. T3 Time when message was sent by responding node. T4 Time whenmessage was received by requesting node. RTT Round-trip-time RTTMIN Thesmallest RTT value during a synchronization phase. CABS0 Wall clock of asynchronization message in the P1 phase CDIFF0 The difference/offsetbetween the two clocks at a synchronization message in the P1 phaseCABS1 Wall clock of a synchronization message in the P3 phase CDIFF2 Thedifference/offset between the two clocks at a synchronization message inthe P3 phase SEQ Sequence number set by requesting node. A[ ] Vectorcontaining T1 for all measurement messages. B[ ] Vector containing T2for all measurement messages. L[ ] Vector containing the resultingone-way latencies, or the latency profile. Ks Overhead of sending amessage Kr Overhead of receiving a message RATEBIAS Difference in ratebetween the two clocks VALID[ ] Vector of boolean values determining thevalidity of the entries in A[ ], B[ ] and L[ ]

The output of the present invention is a latency profile, which is thevector containing the resulting one-way latencies, or L[ ].

In FIG. 4, an embodiment of a requestor node pre-synchronizationflowchart is schematically depicted. The node sends a syncreq to theresponding node. It sets the sequence number and the T1 timestamp in thesyncreq message. Then it waits for a reply to come back from theresponding node, or for a timeout to occur. If a syncreq message wasreceived, a timestamp T4 is registered when the syncresp message wasreceived. Together with the three timestamps T1, T2 and T3, the moduletries to find the message with the smallest round-trip-time. Thismessage is used to find the two values CABS0 and CDIFF0 and is used inthe interpolation method P4. The method uses two variables NSREQ andNSRESP to record the number of sent syncreq messages and receivedsyncresp messages, respectively. These variables are used as aterminating condition. If the module sends 2SNR syncreq messages withouthaving received SNR syncresp messages, this is an error. As soon as themodule has received SNR syncresp messages, it continues to the nextphase, P2A. SNR is a predefined constant, typically 50 messages. Themethod may also use the variables RTT and RTT_MIN. RTT_MIN is preset toa large value, and is used to find the syncreq/syncresp pair with thesmallest round-trip-time. This measurement is then used to compute theCABS and CDIFF values. In other words, we claim that the bestmeasurement is the one with the smallest RTT. Many other methods use themean value. Note that the method described in FIG. 4 may be implementedsomewhat differently. For example, the sending and receiving of messagescan be made concurrently, not sequentially as is shown in the figure. Inthat case, two processes are created, one that is sending syncreqmessages regularly, and one that is waiting for syncresp messages. Inthat case, a timeout need not be made. Instead, a delay between thesending of syncreq messages need to be introduced.

In FIG. 5, an embodiment of a responder node pre-synchronizationflowchart is schematically depicted. The node waits for a syncreq fromthe requesting node. When such a message is received, it creates asyncresp message, copies the sequence number and T1 from the syncreqmessage, records T2 and T3, and sends the syncresp message back to therequesting node. If the received message is not a syncreq message, it isassumed that it is a measurement message which is handled in P2B. Thesize of the vectors is equal to the number of measurement messages sent.

The measurement phase consists of the requesting node periodicallysending measurement messages to the responding node. The responding noderecords the timestamps of the time of sending and the time of receivingthe messages in two vectors A[ ] and B[ ], respectively. The size of thevectors is equal to the number of measurement messages sent, NM. The twovectors are later used in P4.

In FIG. 6, an embodiment of a flowchart of requesting node in themeasurement phase is schematically depicted. The requesting node sendsNM messages (for example 10000) with interval DT between each packet(for example 20 ms). Each syncreq message will contain SEQ, the sequencenumber; and T1, the time the message was sent. The overhead of sending amessage Ks is computed initially. This is the difference in time fromwhen the timestamp was taken and when the message was actually sent. Ksmay be set to 0 if the node lacks the capability to compute this time.

In FIG. 7, an embodiment of a flowchart of the responding node is shown.The responding node stores the sending timestamp T1 in a vector A, andthe receiving timestamp T2 in the vector B. The sequence number is usedas an index in the vector. The overhead of sending a message Kr iscomputed initially. This is the difference in time from when thetimestamp was taken and when the message was actually sent. Kr may beset to 0 if the node lacks the capability to compute this time.

The second synchronisation phase is in this embodiment similar to phaseP1 described above. The differences are as follows:

-   -   1. The two processes are called P3A and P3B instead of P1A and        P1B, respectively.    -   2. The resulting variables are named CABS1 and CDIFF1 instead of        CABS0 and CDIFF0, respectively.    -   3. After successful completion of the processes, both flowchart        goes to P4 instead of to P2A and P2B.

In the interpolation phase, the measurements collected in phase P2 inthe vectors A[ ] and B[ ] and the synchronization values CABS0, CDIFF0,CABS1 and CDIFF1 in phases P1 and P3 are used to interpolate a sequenceof one-way latency values. The method itself can be performed on therequesting node, the responding node, or some other node, and can beperformed at any time after the other three phases. For example, thisphase can be made as a post processing stage in a server. However, thedata must be transferred to the place where the method is implemented.The end result of the method is a vector L[ ], i.e. the latency profile,with size NM containing the true one-way latency values of themeasurement between the requesting and responding node.

In FIG. 8, an embodiment of a flowchart of the interpolation method isschematically depicted. First the difference in rate RATEBIAS iscomputed as follows:RATEBIAS=(CDIFF1−CDIFF0)/(CABS1−CABS0),

The method iteratively computes the values of the one-way latency vectorL[ ] from values collected or computed, as follows:L[i]=B[i]−(A[i]+CDIFF0+RATEBIAS*(A[i]−CABS0))−Ks−Kr

The invention claimed is:
 1. A system for determining one-way latencybetween a first computer node (A) having a first clock and a secondcomputer node (B) having a second clock, comprising: a network modulerunning on said first computer node (A) to pre-synchronize said firstclock of said first computer node (A) with said second clock of saidsecond computer node (B), said pre-synchronizing further comprisingobtaining a first clock difference value and a first absolute clockvalue; measure a sending time and a receiving time associated with eachmeasurement message within a plurality of measurement messages;post-synchronize said first clock of said first computer node (A) withsaid second clock of said second computer node (B), saidpost-synchronizing further comprising obtaining a second clockdifference value and a second absolute clock value; performinterpolating, comprising adjusting the sending time associated witheach measurement message, said adjusting based on the sending time, thesaid first and second clock difference values, and the said first andsecond absolute clock values, further wherein said adjusting comprisescalculating a ratebias, and  said calculating of ratebias comprisingcalculating a denominator; and calculate the one-way latency associatedwith a measurement message in the data network between said firstcomputer node (A) and said second computer node (B) as the differencebetween the receiving time associated with the measurement message, andthe adjusted sending time associated with the measurement message; andgenerate a latency profile.
 2. The system of claim 1, wherein saidadjusting further comprises calculating a shift.
 3. The system of claim2, wherein said shift is calculated using said ratebias, said firstclock difference value, and said first absolute clock value.
 4. Thesystem of claim 3, wherein said shift is calculated further based on thesending time associated with a measurement message and an overheadrequired to send the measurement message.
 5. A system to generate aone-way latency profile between a first computer node (A) having a firstclock and a different second computer node (B) having a second clock,located in a data network without continuous clock synchronization,comprising: a network module running on said first computer node (A) topre-synchronize said first clock of said first computer node (A) withsaid second clock of said second computer node (B) to generatepre-synchronization values representing one or more absolute clockvalues and one or more clock difference values, and obtain a firstabsolute clock value and a first clock difference value from said one ormore absolute clock values and one or more clock difference values, saidpre-synchronizing carried out using a plurality of synchronizationrequest and synchronization response messages; collect, following saidpre-synchronizing, a predetermined number of time measurements, using aplurality of measurement messages; post-synchronize, following saidcollecting, said first clock of said first computer node (A) with saidsecond clock of said second computer node (B) to generatepost-synchronization values representing one or more absolute clockvalues and one or more clock difference values, and obtain a secondabsolute clock value and a second clock difference value from said oneor more absolute clock values and one or more clock difference values,said post-synchronizing carried out using a plurality of synchronizationrequest and synchronization response messages; interpolate saidplurality of time measurements to create a predetermined number oflatency measurements, said interpolating comprising adjusting said timemeasurements using said first and second absolute clock values and saidfirst and second clock difference values, wherein said adjustingcomprises calculating a ratebias, and  said calculating of ratebiascomprising calculating a denominator; and generate said one-way latencyprofile based on said latency measurements.
 6. The system of claim 5,wherein said adjusting further comprises calculating a shift.
 7. Thesystem of claim 6, wherein said shift is calculated using said ratebias,said first clock difference value, and said first absolute clock value.8. The system of claim 7, wherein said shift is calculated further basedon the sending time associated with a measurement message and anoverhead required to send the measurement message.